It is well known that the velocity of an air flow close to the surface of an airfoil or other object is significantly less than the velocity of the air flow at a spaced-apart location from the object, whereat the velocity is measured relative to the object. The region of reduced air flow velocity along and near the surface, termed the boundary layer, is the result of viscous drag between the air flow and the surface. In contrast, the region of air farther from the surface where the velocity is substantially unaffected by the surface is called the free stream.
To achieve optimum performance, modern aircraft propulsion systems, namely jet engines, require an incoming flow of air at the propulsion nacelle, or pod inlet, which is substantially lacking in a boundary layer. The problem with this requirement is that the inlet of a propulsion nacelle is often located adjacent to another surface of the associated airplane. This is particularly true in high velocity (i.e., supersonic) airplanes, such as commercial supersonic transport airplanes, military jet fighters, combat support airplanes, and bombers. When the air flows across a surface adjacent to the inlet of a propulsion nacelle, a boundary layer of lower velocity air is formed. The lower velocity boundary layer air, along with higher velocity free stream air, flows into the inlet of the propulsion nacelle. As a result, some of the air flow into the engine is boundary layer air. Because the momentum of boundary layer air is lower than the momentum of free stream air, engine performance is decreased.
One attempted solution to this problem has been to space the inlet of the propulsion nacelle away from the adjacent surface a distance approximately equal to the thickness of the adjacent surface boundary layer using a pylon or strut, termed a diverter. Thus, the boundary layer impacts the diverter, rather than entering the inlet. Typically the diverter is aerodynamically contoured to smoothly redirect the boundary layer away from the inlet.
Although satisfactory in some cases, the use of a pylon or strut diverter has two principal drawbacks. First, the diverter increases drag by increasing the cross-sectional area presented to the oncoming air flow. Second, at supersonic speeds, the diverter is especially disadvantageous because a shock wave forms in front of the diverter, which significantly increases drag.
Another proposed solution is shown in U.S. Pat. No. 3,578,265 to Patierno et al. Patierno et al. disclose slots formed through the portion of the wing that attaches to the fuselage of an airplane. Boundary layer air flowing along the fuselage passes through the slots, rather than into the propulsion nacelle inlets, wherein the propulsion nacelle inlets extend outwardly from the fuselage, underneath each wing.
There are several drawbacks to the proposed solution described in the Patierno et al. patent. First, the slots reduce the maximum lift available from the wings. Second, while some of the boundary layer air flowing along the fuselage may pass through the slots, a significant portion still flows into the propulsion nacelle inlets. Third, the solution only works with airplanes having propulsion nacelle inlets positioned underneath the wings. Finally, related to the last point, no provision is made for boundary layer air flowing along the underside of the wings. Provision is only made for boundary layer air thinning along the fuselage.
Another proposed solution, often termed boundary layer bleed, is to form apertures in the vehicle surface and apply a partial vacuum to the apertures. The vacuum draws boundary layer air into the interior of the vehicle through the apertures, rather than allowing the boundary layer air to flow into the propulsion nacelle inlet. Alternatively, the apertures are formed such that pressure applied to the apertures causes high velocity air to be injected along the vehicle surface, often termed boundary layer blowing. The high velocity air mixes with the boundary layer, re-energizing the boundary layer. A disadvantage with both of these approaches is that ultimately the engine must supply the energy required to apply vacuum or pressure to the apertures. Thus, any increase in engine performance produced by either approach is offset by the additional energy required to be produced by the engine.